Magnetic latch and switch using cobalt-rare earth permanent magnets

ABSTRACT

Magnetic latches and switches based on the flux cancellation or flux diversion principle use the high coercive force of cobaltrare earth permanent magnets such as cobalt-samarium. These permanent magnets are not demagnetized by a flux cancellation coil and can be made thin in the field direction. Devices with thin magnetic circuits and low volume armature achieve high unlatching speeds.

United States Patent Edgar et al. [4 1 June 20, 1972 s41 MAGNETIC LATCHAND SWITCH 3,146,381 8/1964 Moreau .335/229 USING COBALTLRARE E ARTH3,089,064 5/1963 De Bennetot... ..335/229 3,540,945 1 1/1970 Stmat 6t81. ..148/31.57

lnventors: Robert F. Edgar; Francois D. Martzlofi, both of Schenectady;Russell E. Tompkins,

Scotia, all of NY.

Assignee: General Electric Company Filed: Nov. 18, 1970 Appl. No.:90,676

US. Cl ..335/l79, 335/229 Int. Cl. ..H0lh 51/27 Field of Search..335/170, 167, 166, 179, 229-230,

References Cited UNITED STATES PATENTS Stockl "335/230 PrimaryExaminer-Harold Broome Attorney-John F. Ahem, Paul F. Frank, Julius .l.Zaskalicky, Donald R. Campbell, Frank L. Neuhauser, Oscar B. Waddell andJoseph B. Forman 1 1 ABSTRACT Magnetic latches and switches based on theflux cancellation or flux diversion principle use the high coerciveforce of cobalt-rare earth permanent magnets such as cobalt-samarium.These permanent magnets are not demagnetized by a flux cancellation coiland can be made thin in the field direction. Devices with thin magneticcircuits and low volume armature achieve high unlatching speeds.

4 Claims, 8 Drawing figures PATENTEDJHM20 I972 SHEET 10F 2 INVENTORSROBERT F. EDGAR H (kilo-oersted) FRANCOIS D. MARTZLOFF RUSSELL E.TOMPKINS BY ham ,4 e THEIR ATTOR EY PATENTEflmzo I972 3, 671 ,893

SHEET 20F 2 INVENTORS ROBERT F. EDGAR FRANCOIS D. MARTZLOFF RU zTOMPKINS 7535511. BY M24 THEIR ATT RNEY BACKGROUND OF THE INVENTION Thisinvention relates to magnetic latches and switches made with cobalt-rareearth permanent magnets such as the cobalt-samarium magnet, and moreparticularly to new constructions made possible by the specialproperties of these rare-earth permanent magnets.

In equipment of many different types a desirable component is africtionless latch that is easily and dependably released by anelectrical signal. Magnetic or electromagnet latches are commonlyemployed to accomplish this function. Basically, the magnetic latchcomprises an armature held by magnetic forces against the pole faces ofa magnetic circuit. The magnetic field is temporarily cancelled orreduced, allowing the armature to move away from the pole pieces underthe influence of a mechanical force, usually a spring or gravity. Twoelementary approaches used to neutralize temporarily the magnetic fieldof the magnet are flux cancellation and flux switching. In fluxcancellation systems, the holding flux is provided by an electromagnetwhose field is momentarily cancelled by an opposing field generated by acontrol electromagnet. The holding field in practical devices presentlyknown usually is not provided by a conventional permanent magnet becausethe cancelling field would de-magnetize it. In flux switching systems,the holding flux provided by a permanent magnet or an electromagnet isdiverted from the armature by establishing in a shunt magnetic path aflux which subtracts from the armature flux but reinforces the magnetflux. Although there is no demagnetization of the permanent magnet, ifused, the diverting shunt path is space consuming.

The present invention recognizes that the unique properties of therelatively new cobalt-rare earth permanent magnets make possible new andimproved magnetic latch constructions, and that these concepts havegeneral utility in similarly constructed switches and relays. Thecobalt-rare earth permanentmagnets were first described to the public in1967, and subsequent interest in them has generally been with regard totheir preparation and properties rather than with regard to theirapplications.

SUMMARY OF THE INVENTION The cobalt-rare earth permanent magnets arecharacterized by a high coercive force, H and medium values of magneticinduction, B. These permanent magnet materials, of which cobalt-samariumisthe most common, are more specifically comprised substantially of COR, where R is a rare earth metal. The demagnetization curve of Co Sm islinear with a typical H of 8,000 oersted and a remanent magnetization,B,, of 8,000 gauss. These unique properties of the cobalt-rare earthpermanent magnets, principally the high coercive force, are utilized inthe construction of new and improved magnetic latches and electricalswitches based on either the flux cancellation or flux diversionprinciple.-

In one embodiment, a magnetically operated device of this type includesa magnet assembly comprising a cobalt-rare earth permanent magnet and aflux cancellation coil, preferably wound about the magnet, thatselectively produces a magnetic field opposed to that of the permanentmagnet. A ferromagnetic armature normally is in latched positionmagnetically attracted to the magnet assembly but is movable to releasedposition under the influence of a biasing force, such as spring force orgravity. A circuit energizes the coil at least temporarily to cancel themagnet flux completely or partially, to thereby reduce the magneticforce acting on the armature and release it for movement to the releasedposition. The permanent magnet is not demagnetized and the armature isrelatched upon return by a restoring means. Built with thin magneticcircuits and a relatively thin armature, the device has high unlatchingspeed with modest electrical requirements and has application forexample as a print actuator in a high speed printer. The constructionhas general application, however, without size and speed limitations.

In a second embodiment, a multipole device based on the flux diversionprinciple is similar but utilizes coils, not wound on the magnets, thateach selectively produces a magnetic field with the same polarity as theassociated cobalt-rare earth permanent magnet. The principal value ofthis embodiment is the low volume, high speed armature made possible bythe use of thin magnetic circuits.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic perspectiveview of a high speed latch based on the flux cancellation principle andconstructed in accordance with the invention with a cobalt-sarnariumpermanent magnet and an encircling flux cancellation coil;

FIG. 2 is a schematic cross-sectional view of FIG. 1 illustrating infull lines the latched position of the armature attracted against thepole faces of the magnetic circuit and in dotted lines the releasedposition, further showing a schematic circuit diagram of the fluxcancellation coil pulsing circuit; I

FIG. 3 shows the demagnetization curves of several permanent magnetmaterials including that of cobalt-samarium to illustrate thesuperiority of this new material;

FIG. 4 is a fragmentary perspective view, with portions broken away, ofan illustratory application of the new magnetic latch of FIGS. 1 and 2in the hammer actuator of a high speed printer;

FIG. 5 is a diagrammatic perspective view of a normally open switch,illustrated with the armature released to close the circuit, which isbased on the principles of the permanent magnet flux cancellation latchof FIGS. 1 and 2;

FIG. 6 is a diagrammatic perspective view of a multipole latch based onthe flux switching principle and constructed in accordance with theinvention with thin magnetic circuits employing cobalt-samarium magnets;

FIG. 7 is a schematic cross-sectional view of FIG. 6, with theDESCRIPTION OF THE PREFERRED EMBODIMENTS The single pole magnetic latchshown in FIGS. 1 and 2 comprises essentially a rectangularcobalt-Samarium permanent magnet 11 mounted between a pair of similarlyshaped but larger soft ferromagnetic pole pieces I2 and 13. A fluxcancellation coil 14 is wound directly about permanent magnet 11 in theremaining peripheral space between the two pole pieces. Fluxcancellation coil 14 is connected to a suitable pulsing circuit such asthe capacitor discharge circuit in FIG. 2, which includes a capacitor 15charged through resistor 16 from a battery 17 or other low energysource, coil 14 being connected across the capacitor through a switch18. The light-weight, relatively thin armature 19 is also made of a softferro-mag netic material, such as soft iron or cobalt-iron. Armature I9is normally attracted against the pole faces of pole pieces 12 and 13(FIG. 2), but upon operation of the latch drops by gravity to itsreleased position supported on plate 20. To reset the latch, push rods21 and 22 extending through plate 20 are elevated to restore armature 19to its normal position attracted toward pole pieces 12 and 13. Theshowing of the magnetic latch in FIGS. 1 and 2 is, of course, highlyschematic and many other arrangements are possible. In general, armature19 after release moves away from the pole pieces 12 and 13 under theinfluence of a biasing mechanical force, commonly gravity or a springforce. The restoring force for the armature 19 is held by the forces ofmagnetic attraction against pole pieces 12 and 13 and completes amagnetic circuit for the Y flux from permanent magnet 11. The poles ofmagnet 11 and pole pieces 12 and 13 have the polarity indicated. Whenthe flux in the air gap between the armature and the poles is reduced,the magnetic force decreases according to the square of the flux and thearmature is released. The flux reduction is obtained by momentarilypulsing flux cancellation coil 14 by closing switch 18 to dischargecapacitor 15 through the coil. The magnetic field produced by coil 14 isopposite in polarity to the field of permanent magnet l 1 and partiallyor complete- 1y cancels the magnet field so that armature 19 isreleased. An important feature of the invention is that, due to the highcoercive force of the cobalt-samarium magnet material, permanent magnet11 is not permanently demagnetized by the oppositely poled fluxcancellation coil field.

Another important feature of the invention is that cobaltsamariumpermanent magnet 11 can be made relatively thin in the direction of thefield. That is, the dimension L can be made small, in the order. ofmils. The unique property of cobalt-samarium that permits the use ofextremely thin magnetic circuits in the field direction is again thehigh coercive force of the cobalt-rare earth permanent magnets, incombination with their medium flux density capability. Pole pieces 12and 13 can also be made relatively narrow in the same direction,depending on the flux, and the important consequence follows that thearmature can have a relatively small thickness t. A small, low volumearmature has the advantage of high speed unlatching operation. As oneillustration of the small size that can be attained, the length L ofcobalt-samari um magnet 11 is 20 mils, while the over-all length L ofthe magnet and pole pieces is 90 mils. For these dimensions thethickness t of armature 19 is on the order of 20 mils. For a given areaof magnet 11, the cross section of pole pieces 12 and 13 is madesufficiently large to obtain the required flux density in the polepieces. For the example given, the area coordinates are that the heightof the pole pieces is about 100 mils while the depth is about 1 inch,and armature 19 is 90 mils by 1 inch. The power requirements for fluxcancellation coil 14 are modest and consistent with the integratedcircuit approach. Furthermore, a small width pulse can be used toenergize coil 14 since, due to the small dimensions of pole pieces 12and 13 in the order of tens of mils, armature 18 is released after a fewmils of travel away from the pole faces. For the example given, thecapacitor 15 is a 25-volt, 50;.LF, capacitor.

The special properties of the cobalt-Samarium permanent magnet that makeit suitable for a high speed, flux cancellation type magnetic latch arebetter understood by reference to the demagnetization curves shown inFIG. 3 for this permanent magnet material as well as some other commonpermanent magnet materials. The demagnetization curves are moreparticularly the B-I-I characteristics, where B is the magneticinduction in kilogauss and H is the magnetizing force in kilo oersted.To enable a comparison the demagnetization curves for alnico, bariumferrite, cobalt-platinum, and cobalt-samari-' urn (Co Sm) are shown.Actually, a family of alnico and cobalt-platinum curves are shown, wherethe particular curve to be used depends upon the proportions of theconstituent metals in the magnet. The property of special interest isthe coercive force H defined as the magnetizing force required to bringthe flux density to zero in a magnetic material that has been magnetizedalternately by equal and opposite magnetizing forces. It is the value ofH when B is zero, that is, the reverse magnetizing force needed toremove the residual magnetism. It is seen that the demagnetization curveof cobaltsamarium is linear, with a remanent magnetization value B, of 8kilogauss and a coercive force I-I of 8 kilooersted. Consequently,cobalt-Samarium is characterized by a very high coercive force andmedium flux density values. By comparison, the alnico alloys are capableof supporting high flux densities but the material has a low coerciveforce. The cobaltplatinum alloys are characterized by a medium coerciveforce and medium flux density. The barium-ferrite curve is also linearlike that of cobalt-Samarium, which means that these materials can bevaried throughout their entire curve without permanent demagnetization,but the considerably higher coercive force and flux density obtainablewith cobalt-samarium is obvious. As was previously mentioned, thecoercive force is the property that permits the use of flux cancellationcoil 14 without demagnetizing the rare earth magnet 11. The relativelyhigh coercive force, in combination with the medium flux densitycapability, permits the use of thin samples of material since the energyproduct, BI-I, has a relatively high value because of the high coerciveforce.

In addition to the cobalt-samarium permanent magnets, other cobalt-rareearth permanent magnet materials useful in the practice of the inventionare cobalt-yttrium and cobaltmisch metal. Misch metal is the most commonalloy of the rare earth metals which contains the metals in theapproximate ratio in which they occur in their most common and naturallyoccurring ores. These new permanent magnet materials are describedfurther in an article by Stmat et al., Journal of Applied Physics (38),1967, pp. 1001-2. Also see the copending patent application entitled"Liquid Sintered Cobalt-Rare Earth Intermetallic Products" by Mark G.Benz, Ser. No. 33,347, filed Apr. 30, 1970, and assigned to the GeneralElectric Company. In the latter application it is explained thatcobalt-rare earth intermetallic compounds exist in a variety of phases,but the Co R intermetallic single phase compounds, where in eachoccurrence R designates a rare earth metal, have exhibited the bestmagnetic properties. Consequently, a more specific description of thepermanent magnet materials useful in the practice of the invention is amaterial comprised substantially of Co R, where R is a rare earth metal.

In FIG. 4 there is illustrated one application of the new and improvedmagnetic latch of FIGS. 1 and 2 in the printing mechanism of a highspeed printer. A row of character positions each includes acobalt-samarium magnet assembly including the two pole pieces andencircling flux cancellation coil. A reed spring 25 functions as acombination armature and spring force for moving the armature toreleased position when the flux cancellation coil is pulsed. Upon beingreleased, a pressure pad 26 on the other side of the reed engages acylindrical hammer 27 slidable horizontally in a guide block 28. On theother side of guide block 28 are located the ink ribbon 29, the paper 30being printed, a plurality of parallel print fingers 31 each with araised print character at the upper end, and a backup plate 32. Uponpulsing the flux cancellation coil in cobalt-Samarium magnet assembly24, reed spring 25 is released and pushes forward hammer 27 to strikethe ribbon 29 and paper 30 upon print finger 31, thereby printing acharacter on paper 30.

The advantages of the hammer actuator in FIG. 4 are the high density ofthe print positions and the high unlatching speed that contributes toachieving a high speed printer. The spacing of adjacent cobalt-samariummagnet assemblies 24 and reed springs 25, and consequently the printfingers 31 can be as small as 0.1 inch. High speed printing is madepossible by the fact that the reed spring armatures 25 have a low massand are released within a very short interval of time. To return reedsprings 25 to the latched position, the base of each reed spring isrotated and the tip of each spring is wrapped against the curved face ofmagnet assembly 24 by a restoring force such as an elongated barindicated diagrammatically by arrow 33.

Referring to FIG. 5, another application of the cobalt-rare earthmagnetic latch of FIGS. 1 and 2 is as an integral part of a switch orrelay. The electrical switch shown in FIG. 5 is a normally open switch,more specifically a switch that is closed in the released position ofthe armature. As in FIG. 4, the armature is combined with the springforce that moves it to released position. Reed spring 34 is made of asoft ferromagnetic material and is anchored at one end to a stationarysupport 35. At the free end is a strip 36 of conductive metal, such ascopper, that is engageable with a pair of stationary contacts 37 and 38to complete the circuit for the flow of current. In operation, fluxcancellation coil 14 is pulsed when it is desired to close the switch,releasing spring 34 to flex outwardly and engage conductive strip 36with contacts 37 and 38 as just described. To reset the switch, amechanical force as for instance a cam or pawl 39, engages the end ofspring 34 and moves the spring back down to the latched positionadjacent the pole faces of pole pieces 12 and 13.

The cobalt-samarium magnetic latch is usable in a variety of switch andrelay configurations, only one of which is illustrated. The switch shownin FIG. 5 has the particular advantages of high unlatching or switchingspeed and small size. Normally closed as well as normally openedconfigurations are possible. Larger sizes of switches and relays arewithin the scope of the invention, also, limited only by the practicalrestrictions on the size of the flux cancellation coil needed to canceltemporarily the holding field due to the cobalt-rare earth permanentmagnet.

Another embodiment of the invention illustrated in FIGS. 6-8 is a highspeed multipole cobalt-rare earth magnetic latch based on the fluxdiversion principle. A row of spaced cobaltsamarium permanent magnets40a-40d are sandwiched between a plurality of soft iron pole pieces4la-41e. Alternate permanent magnets 40a-40d are oppositely poled, sothat the pole faces of the pole pieces 4la-41e have opposite polarity.Flux diversion coils 42a-42d are mounted in the space above thepermanent magnets between the opposing pairs of pole pieces. For thereasons already given in connection with the magnetic latch constructionof FIGS. 1 and 2, cobalt-samariurn magnets 400-4011 and pole pieces4la-41e can be made relatively thin. Although soft iron armature 43 hassufficient area to cover the entire gap surface of the magnet assembly,it can have a small thickness. Consequently, armature 43 has low weight,low mass and inertia, and is capable of high speed unlatching movement.As with the single pole latch, the normal position of armature 43 islatched against the pole faces of pole pieces 4la-41e as shownin FIG. 7.Electrically, the flux diversion coils 42a-42d are preferably connectedin parallel branches (FIG. 8), and are connected to be energized by asuitable pulsing circuit such as the capacitor discharge circuitpreviously explained with regard to FIG. 2. Closure of switch 18momentarily pulses all four flux diversion coils 42a-42d at the sametime.

The momentary energization of flux diversion coils 42a-42d results inthe creation by each coil of a magnetic field which reinforces theassociated magnet flux but subtracts from the flux path to the armaturethrough the associated pole pieces. Therefore, the magnetic forcesattracting armature 43 to its latched position are reduced, and thespring forces of springs 44 and 45 move armature 43 to its releasedposition abutting stops 46 and 47. Push rod 48 extends through thesupport 49 on which the springs are mounted, and supplies the restoringforce to return armature 43 from its released position to its latchedposition. The advantages of low armature mass and high unlatching speedobtained by the new multipole cobaltsamarium magnetic latch have alreadybeen mentioned. It is obvious that pole pieces 41a-41e and armature 43can be made of any suitable soft ferromagnetic material and that othercobalt-rare earth permanent magnets can be substituted forcobalt-samarium.

In summary, new and improved magnetic latches, switches, and relays aremade possible by the unique properties of the cobalt-rare earthpermanent magnets, which are more particularly described as materialscomprised substantially of C0,,R,

, where R is a rare earth metal such as samarium. Principally because ofthe high coercive force, high speed devices with low volume armatures,based on either the flux cancellation principle or the flux diversionprinciple, are constructed with thin magnetic circuits incorporatingpermanent magnets. They further have modest electrical control powerrequirements. However, similarly constructed devices with moresubstantial magnetic circuits and power requirements are within theteaching of the invention.

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A high speed magnetically operated device comprising a magnetassembly including a cobalt-rare earth permanent magnet mounted betweena pair of pole pieces having oppositely poled pole faces, said permanentmagnet and pole pieces having a relatively thin field directiondimension as compared to the area coordinate dimensions, and a fluxcancellation coil wound about said permanent magnet that selectivelyproduces a magnetic field opposed to the polarity of said permanentmagnet,

a low volume ferromagnetic armature attracted by magnetic forces to alatched position against the pole faces of said magnet assembly, andmovable under the influence of a biasing force to a released position,said armature having a relatively small thickness dimension as comparedto the area coordinate dimensions,

circuit means for temporarily applying only one polarity of directcurrent to said flux cancellation coil to thereby reduce the magneticforces acting on said armature and release said armature for movement tothe released position, and

restoring means for returning said armature to the latched positionmagnetically attracted to said magnet assembly.

2. A device according to claim 1 wherein said armature is a reed springthat supplies the biasing force for movement to the released position.

3. A device according to claim 2 wherein said device is an electricalswitch and further includes a pair of contacts engaged by said reedspring armature in the released position.

4. A high speed magnetically operated device comprising a multipolemagnet assembly including a plurality of linearly arranged alternatingcobalt-rare earth permanent magnets and ferromagnetic pole pieces havingalternately oppositely poled pole faces, and a plurality of fluxdiversion coils each mounted between a pair of adjacent pole pieces thateach selectively produces a magnetic field with a polarity to shunt themagnetic flux produced by the associated permanent magnet mountedbetween the same pair of pole pieces,

a ferromagnetic armature attracted by magnet forces to a latchedposition against the pole faces of said magnet assembly, and movableunder the influence of a biasing force to a released position, whereinsaid cobalt-rare earth permanent magnets and pole pieces have arelatively thin field direction dimension as compared to the areacoordinate dimensions, and said armature is a low volume armature with arelatively small thickness dimension as compared to the area coordinatedimensions,

circuit means for temporarily applying only one polarity of directcurrent to said flux diversion coils to reduce the magnetic forcesacting on said armature and release said armature for movement to thereleased position, and

mechanical restoring means for returning said armature to the latchedposition magnetically attracted to said magnet assembly.

k t a: 1k

2. A device according to claim 1 wherein said armature is a reed springthat supplies the biasing force for movement to the released position.3. A device according to claim 2 wherein said device is an electricalswitch and further includes a pair of contacts engaged by said reedspring armature in the released position.
 4. A high speed magneticallyoperated device comprising a multipole magnet assembly including aplurality of linearly arranged alternating cobalt-rare earth permanentmagnets and ferromagnetic pole pieces having alternately oppositelypoled pole faces, and a plurality of flux diversion coils each mountedbetween a pair of adjacent pole pieces that each selectively produces amagnetic field with a polarity to shunt the magnetic flux produced bythe associated permanent magnet mounted between the same pair of polepieces, a ferromagnetic armature attracted by magnet forces to a latchedposition against the pole faces of said magnet assembly, and movableunder the influence of a biasing force to a released position, whereinsaid cobalt-rare earth permanent magnets and pole pieces have arelatively thin field direction dimension as compared to the areacoordinate dimensions, and said armature is a low volume armature with arelatively small thickness dimension as compared to the area coordinatedimensions, circuit means for temporarily applying only one polarity ofdirect current to said flux diversion coils to reduce the magneticforces acting on said armature and release said armature for movement tothe released position, and mechanical restoring means for returning saidarmature to the latched position magnetically attracted to said magnetassembly.